The present invention relates generally to a method and apparatus for controlling a hydraulically powered seismic vibrator, and more particularly relates to a method and apparatus for controlling the pressure of the hydraulic fluid driving the vibrator so as to improve the quality of the signal imparted to the earth by the vibrator.
Hydraulically powered vibrators are commonly used in seismic exploration to impart energy into the earth. A typical vibrator comprises a piston and cylinder assembly mounted on a truck such that the piston reciprocates in a vertical direction. The piston is adapted to contact a baseplate which is carried by the truck and which is adapted to contact the earth's surface. The piston is typically operatively connected to the baseplate such that vibration of the piston is transmitted through the baseplate to the earth.
The cylinder housing carried by the truck is a heavy mass, called a reaction mass, utilized to help assure good coupling between the baseplate and the earth. The cylinder housing often weighs several thousand pounds. When a seismic observation is to be made, the baseplate is placed in contact with the ground. The truck is then often jacked off the ground so that the truck's mass assists in coupling the baseplate to the earth.
In operation of the vibrator, an electrical signal is utilized to establish a reciprocating motion of the vibrator piston. Typically, this electrical signal will be a sinusoidal signal. In a typical vibrator, a control valve directly responsive to the sinusoidal electrical signal directs hydraulic fluid alternately to one side of the piston, to cause movement in a first direction, and then to the alternate side of the piston to cause movement in the reverse direction. The control valve typically is a spool valve in which a central valve spool moves back and forth within a valve body. The movement of the valve spool between opposite extremes of travel causes the above-described direction of hydraulic fluid to alternating sides of the piston.
As the valve spool travels from one end position to the other end position in response to the drive signal, the valve spool passes through a null point in which the hydraulic fluid is trapped in the piston/cylinder assembly. The trapping of this fluid when the valve spool passes through the null point causes a spike or pulse to occur in the pressure across the piston. This pressure spike produces distortion in the force which the vibrator imparts to the earth. Specifically, the signal will produce a series of unwanted harmonic peaks in the frequency spectrum of such force. This pressure spike is typically largest when the valve spool is reciprocating at relatively low frequencies. If certain harmonics have too great an amplitude, the vibrator baseplate may decouple from the earth. Such decoupling not only adversely affects the energy imparted to the earth, but also increases the mechanical strain on the various parts of the vibrator.
In some hydraulic seismic vibrators, it is desirable to utilize various electronic signals to optimize control of the force generated by the vibrator and imparted to the ground. The signals utilized for such force control may include signals representative of various vibrator response parameters, including the acceleration of the reaction mass (the vibrator cylinder), the acceleration of the baseplate, and the vibrator output force imparted to the ground. The use of these signals to control the force of a vibrator is descibed in co-pending U.S. patent application, Ser. No. 676,718, filed Nov. 30, 1984, entitled "Automatic Force Control for a Seismic Vibrator" and assigned to the assignee of the present invention. The specification of application Ser. No. 676,718 is hereby incorporated herein by reference for all purposes. Briefly, a force control circuit such as that disclosed in the afore-mentioned patent application modifies the electrical drive signal utilized to drive the control valve of the vibrator so as to conform the vibrator output force signal to a pre-established reference signal. When such a force control system is utilized, the presence of harmonic distortion in the vibrator, output force signal or in the other signals representative of vibrator response parameters will adversly affect the performance of the force control circuit.
Accordingly, the present invention provides a new method and apparatus for providing optimal harmonic characteristics of force imparted to the earth by a seismic vibrator and for thereby facilitating the generation of signals representative of such force and of other parameters of vibrater performance which are optimally free of distortion.